The present invention relates to methods for determining the relative humidity of volumes of air, and in particular to methods for determining the relative humidity of a volume of hot air enclosed within industrial machinery such as veneer dryers and the like.
Wood is often peeled, sawn, or sliced into sheets of a certain thickness which are combined with adhesive resins to form wood products such as plywood. These wood sheets or xe2x80x9cveneerxe2x80x9d sheets are formed in veneer mills and are generally dried in veneer dryers before being adhered to one another.
Plywood veneer dryers are large, heated enclosures through which veneer sheets are transported along a conveyor system. A typical veneer dryer might be 80-100 feet in length. A typical drying temperature in a veneer dryer might be 200xc2x0 C. In these veneer dryers, air is circulated by large fans to remove moisture from the veneer sheets.
These veneer dryers dry wet veneer to an average moisture content that is compatible with the adhesive system being used to bond the veneers. This moisture content is also somewhat dependent upon the end product being manufactured. Plywood, for example, is generally manufactured from veneers having a moisture content of between 3%-8% moisture content by dry weight. The veneers cannot be properly bonded unless they have the correct moisture content.
Given the importance of the moisture content of the veneers, then, it is important to monitor and control the moisture content of the veneers in the dryers. Moisture content can generally be controlled by controlling the temperature of the dryer, the length of time the veneers are allowed to be dried, and the relative humidity of the air within the dryers.
The relative humidity of the air within a veneer dryer is generally controlled by venting hot, humid air from the dryer and replacing it with fresh, cool air. However, venting the hot air from the dryer causes energy losses, and it is desirable to closely monitor the relative humidity so as not to unnecessarily vent an excessive amount of hot air.
To this point in time there has not been developed a suitable, cost effective system and method for determining and monitoring the relative humidity within a veneer dryer.
It is quite common and well known to monitor relative humidity within a lumber kiln using the dry and wet bulb method, which will be very familiar to those with knowledge of wood drying and with psychrometry generally. The relative humidity within a lumber kiln can be determined by obtaining the dry bulb temperature and the wet bulb temperature within the kiln and by thereafter referring to a reference table or a database for the relative humidity given these two temperatures. However, it is well known that the dry and wet bulb method of determining relative humidity cannot be suitably relied upon to provide accurate determinations of relative humidity when the temperature of the air exceeds 100xc2x0 C. (the boiling point of water), as it does in a veneer dryer, since proper wet bulb temperatures cannot be obtained.
Accordingly, relative humidity has not typically been determined in veneer dryers by the dry and wet bulb method. Rather, various expensive and complicated optical and electrical systems have been suggested and employed. Other systems (such as the xe2x80x9cZirconia Oxygen Analyzerxe2x80x9d) measure oxygen content directly and humidity indirectly, but these systems do not provide accurate readings in atmospheric environments like those within a veneer dryer wherein the xe2x80x9cfreshxe2x80x9d air has been contaminated by organic compounds given off by the drying wood, and by byproducts of the combustion of fuels (such as natural gas) used to heat the air within the veneer dryer. So, these oxygen-measuring systems, as expensive as they might be, do not provide very accurate readings within a veneer dryer.
There accordingly remains a need for a relatively simple, inexpensive method for determining the relative humidity of a volume of hot air enclosed within a veneer dryer.
The present invention provides a method for determining the relative humidity of a volume of air having a temperature of 100xc2x0 C. or greater. The method comprises the steps of:
measuring the temperature TV of the volume of air;
determining from TV the saturation vapor pressure psat(V) of the volume of air;
collecting from the volume of air a sample volume of air;
cooling the sample volume of air to a temperature TS below 100xc2x0 C., but above the dew point of the sample volume of air;
determining from TS the saturation vapor pressure psat(S) of the cooled sample volume of air;
determining the relative humidity RHS of the sample volume of air; and
performing the calculation
xe2x80x83RHV=RHSxc3x97psat(S)/psat(V)
to obtain a relative humidity RHV of the volume of air.
In a preferred embodiment, the step of determining the relative humidity RHS of the sample volume of air comprises considering the temperature TS to be the dry bulb temperature of the sample volume of air, and further comprises measuring the wet bulb temperature of the sample volume of air. With these dry and wet bulb temperatures, the relative humidity RHS may be obtained from a reference source.
In a further preferred embodiment of the invention, the method is carried out within a veneer drying system and the relative humidity of the volume of air calculated is that of the air within the dryer. The sample of air removed from the dryer may be returned to the dryer after the calculation of relative humidity is performed.